A 401(k) is a feature of a qualified profit-sharing plan that allows employees to contribute a portion of their wages to individual accounts. Elective salary deferrals are excluded from the employee’s taxable income (except for designated Roth deferrals). Employers can contribute to employees’ accounts. Distributions, including earnings, are includible in taxable income at retirement (except for qualified distributions of designated Roth accounts).

Merriam-Webster Online Dictionary
401(k) (noun)
a retirement account to which employee and employer contribute, on which taxes are deferred until withdrawal, and for which the employee usually selects the types of investments
401(k)
Categories: Income